The following U.S. patents and published PCT Patent Applications are believed to represent the current state of the art in the field of the invention:                U.S. Pat. Nos. 6,095,867; 6,049,881; 5,994,998; 5,960,208; 5,944,831; 5,933,073; 5,923,363; 5,859,596; 5,859,584; 5,835,005; 5,828,293; 5,814,900; 5,810,606; 5,799,196; 5,689,230; 5,686,826; 5,684,826; 5,664,002; 5,572,182; 5,517,172; 5,498,911; 5,491,463; 5,477,091; 5,467,384; 5,455,467; 5,452,344; 5,422,519; 5,406,260; 5,351,272; 5,306,956; 5,192,231; 5,148,144; 5,093,828; 5,066,939; 5,033,112; 5,032,833; 5,021,779; 5,003,457; 4,992,774; 4,973,954; 4,926,158; 4,903,006; 4,885,563; 4,815,106; 4,799,211; 4,755,792; 4,733,389; 4,731,810; 4,692,761; 4,528,667; 4,467,314; 4,367,455; 4,290,056; 4,101,878; 3,500,132; 3,423,521.        
Published PCT Patent Applications WO96/23377 and WO92/17968.
The most important reference is believed to be (3Com) U.S. Pat. No. 5,994,998.
The disclosures of the above patent documents and of copending U.S. Ser. No. 09/365,584, Published PCT Application PCT/IL99/00691—WO 00/41496 and of all other patent documents and publications mentioned in this specification are hereby incorporated by reference.